My father has a new trail boss and he was in a little sand when he went to take off the chain just snapped in half. Is this common and should warranty cover this because my dealer says no.

 

This used to be a common thing with the NON CONCENTRIC DRIVE models. But the CONCENTRIC drive has helped greatly.
I am sure the dealer will not cover this under warranty due to it being a wear item, unless you could PROVE they OVER TIGHTENED it (I DON'T KNOW HOW, its already broke). They would not cover My TORN CV BOOT when the bike was 2 months old.
Did the chain snap or did the master come loose? Either way I am sure it is a fluke.
Make sure when you buy a chain that you go ahead and buy a VERY good aftermarket chain, Do not skimp on the cheep one, it will pay in the long run.

Also make sure your sprokets are not bent or missing teeth. Polaris is also know for the chain to get caught up at the front sproket and crack the gear case. It is very important to MONITOR/ADJ. you chain Lash because they seam to get loose often, but also never over tighten .